In image formation by an electrophotographic method, an electrostatic latent image formed on an image carrier (it is also called as a photoreceptor) is developed with a toner to result in visualization. As this developing method, conventionally, two method s are known depending on the rotating direction of a developing sleeve of the developing unit that rotates while holding the developer and the rotating direction of the photoreceptor. One is a normal-rotation developing method, and the other is a reverse-rotation developing method.
As illustrated in FIG. 1A, the normal-rotation developing method is a developing method in which the rotating direction W1 of the photoreceptor P and the rotating direction W2 of the developing sleeve S are an opposite direction. In this method, in the development area, the surface of the photoreceptor and the developer on the surface of the developing sleeve are brought into contact with each other from the same direction, thereby development is carried out.
On the other hand, as illustrated in FIG. 1B, the reverse-rotation developing method is a developing method in which the rotating direction W1 of the photoreceptor P and the rotating direction W2 of the developing sleeve S are the same direction. In this method, in the development area, the surface of the photoreceptor and the developer on the surface of the developing sleeve are brought into contact with each other from the opposite direction, thereby development is carried out.
The normal-rotation developing method and the reverse-rotation developing method both produce a scavenging phenomenon, that is, a phenomenon which disturbs the toner image developed on the photoreceptor by electrostatically scraping the toner with the carrier particles. In particular, an image defect caused by scavenging is easily produced in the normal-rotation developing method.
A specific example of the above-described image defect caused by the scavenging phenomenon is described by referring to FIG. 2. FIG. 2 is a schematic diagram illustrating an image defect that occurred when printing a document having a solid image G1 adjacent to the rear end of the halftone image G2 with respect to the paper feeding direction A. When such printing is performed, as illustrated in the figure, the halftone image of the rear edge of the halftone image, that is, the boundary W with respect to the solid image is likely to be whitened easily (hereinafter referred to as “lead portion whitening” or “lead portion white spot”). Improvement thereof is required.
The cause of the scavenging phenomenon is the difference in moving speed between the developing sleeve and the photoreceptor. The scavenging phenomenon occurs when the moving speed of the developing sleeve is set higher than the moving speed of the photoreceptor. More specifically, the scavenging phenomenon occurs as follows. When a solid image is developed, a large amount of toner is supplied to the photoconductor, whereby charges remain in the carrier particles on the developing sleeve, and the carrier particles overtake the solid image portion on the photoreceptor. When the carrier particles reach the halftone image part, a scavenging phenomenon occurs because the toner that forms the halftone image is electrostatically scraped off.
As an improvement method for suppressing the occurrence of the scavenging phenomenon, reduction in resistance of carrier particles has been proposed. By making the carrier particles to have low resistance, charge of the carrier particles generated at the time of solid image development tends to disappear easily. As a result, the toner is not scraped off, and the scavenging phenomenon hardly occurs.
However, when the resistance of the initial carrier particles is excessively lowered, the resin layer on the surface of the carrier particles is scraped and the core material having lower resistance is exposed on the surface, so that the resistance excessively decreases and the development nip portion leakage occurs, and a problem arises that image defects are likely to occur (for example, refer to Patent document 1: JP-A 2015-230376).
Therefore, in general, it is difficult to make it compatible with suppression of occurrence of lead portion whitening at an initial stage and suppression of occurrence of development leakage when the photoreceptor is used for a long time.